The in situ production of continuous, jointless concrete pipe in open trenches is now widely accepted because it yields high quality pipe at modest costs. Under one method pipe is produced by employing the bottom of an open trench as the supporting medium for the lower pipe half while special molds are provided to support the upper pipe half until the concrete has hardened. The molds are relatively short lengths of metal sheets having a curved shape complementary to the interior curvature of the pipe.
After the concrete has hardened, the molds must be removed by a workman who enters the pipe, releases the molds and then removes them by dragging them to the open end of the pipe. Methods and apparatus for constructing such pipe are described, for example, in U.S. Pat. Nos. 2,731,698; 3,106,760; 3,113,364; and 3,534,449. U.S. Pat. No. 3,785,759 proposes to delete interior pipe molds for supporting the upper pipe half while the concrete hardens. As far as is known to applicant, the method proposed in the last mentioned patent is technically and/or economically not feasible and has not been employed on a commercial scale.
Although the pipe-forming methods and machines proposed in the patents have resulted in substantial improvements for laying large diameter pipes, they have shortcomings. The need for workmen to enter the pipes to remove the molds limits the production of such pipe to a minimum pipe diameter of at least about 24 inches. Moreover, the removal of the molds in and of itself requires substantial manual labor and is, therefore, expensive.
In addition, it is necessary to store large numbers of pipe molds at the construction site and each mold must be individually handled and inserted into the pipe-forming machine. Again, this involves substantial labor and complicates the construction of the machine. The initial costs as well as the subsequent maintenance of the machine are thereby significantly increased.
The prior art concrete pipe forming methods and machines proposed in the patents have the further drawback of requiring an intermittent cessation of the pipe forming method to permit the removal of pipe molds from the interior of already laid pipe. This significantly reduces the production rate attainable with such prior art pipe forming machines. Additionally, once the molds are removed from the pipe they leave ring-shaped depressions on the interior of the pipe. These depressions adversely affect the hydraulic characteristics of the pipe since they cause turbulence in the medium flowing through the pipe.
Lastly, jointless concrete pipe produced in accordance with the prior art exposed the raw concrete to potentially corrosive fumes, which in time, could damage or destroy the pipe. The problem is particularly acute for sewage drain pipe which releases concrete corroding fumes. In the past, such corrosion was counteracted by coating the finished pipe with a suitable protective agent. This, however, is an expensive task since low cost coating techniques such as dip coating of short pipe sections cannot be employed in seamless, in situ formed pipe.